This invention relates to a spring unit for a keyboard. Electronic calculators, office computers, word processors and other computer devices which are in widespread use, are provided with input operating boards or keyboards having a plurality of keys actuated by finger pressure. The keyboard comprises a plurality of key members, a plurality of controlling spring members and contact point means. So that smooth key touch can be obtained, there is a considerable amount of research and development being conducted on key members, springs, etc. Particularly, since the spring members are operated by finger pressure, they must have suitable resilience and stability so that a smooth and fatigueless key touch is obtained.
Originally controlling spring members were made of a metallic material, but recently they are being made of a high polymer resilient material.
A controlling spring member itself is a single unit. In accordance with recent developments, an array of controlling spring members is integrally formed on a keyboard, an arrangement which provides for good operability. This was made possible because of good workability of the high polymer resilient material.
FIGS. 1 and 2 in the drawings show the conventional art, in which the former is a plan view of a spring unit a for a conventional keyboard and the latter is an expanded partially cutaway section view in which the spring unit a is incorporated in the keyboard.
Referring first to FIG. 1, numeral 1 is a base sheet and numeral 2 is a spring integrally connected with the base sheet 1. Both the base sheet 1 and the spring 2 are made of a high polymer resilient material and are molded integrally as a spring unit a for the keyboard. The spring 2 which is roughly dome shaped has a head 3, on which a key top 4 is mounted. Numeral 5 is a movable contact point which is connected to the lowest part of the head 3.
Accordingly, when the key top 4 is pushed by a finger, the spring 2 is deformed. As the key top 4 is lowered further, the movable contact point 5 of the head 3 completes a circuit of two fixed contact points 8 and 9 of a lower printed circuit substrate 7, thereby transmitting an input signal.
Since the base sheet 1 and a plurality of springs 2 in the spring unit a form an integral structure, it is easy to incorporate the spring unit a in the keyboard 6. A disadvantage is that in the event a particular spring, such as for example 2x, corresponding to a specified key is deformed or inoperative, the entire base sheet 1 itself, being a single unit with its springs, becomes inoperative. Accordingly, the number of base sheets which must be rejected on manufacture increases and the yield rate of satisfactory products is lower, thereby increasing the cost of manufacture.
Further, if the key layout on the keyboard is modified to even a small extent, production of a new mold for the base sheet 1 is required. Additionally, when one wishes to have a specific key having an action property different from that of other keys, one is required either to provide a metal spring separately or to mold an entirely new spring unit a, in which a special key having a different action property is incorporated integrally. In particular, when one wishes to use a special spring 2 having a different action property on the base sheet 1, individual property adjustment for one key is cumbersome and the cost for producing a mold for such a special spring unit becomes very expensive.
Further, if one spring 2 should be inoperative due to abrasion, it would be useless to attempt repair of the spring 2; thus it is required to replace the entire spring unit a with a new one. Under such circumstances, all the springs 2 must be carefully manufactured to have a long life, thus increasing the cost of the keyboard.
As described above, according to the conventional spring unit for a keyboard, a large number of springs are formed integrally on the base sheet. Under these circumstances it is difficult to prescribe a desirable mold manufacturing condition and molding condition for obtaining suitable and uniform resilience of the springs or a desirable vulcanizing condition for the rubber which is used as the material of the spring unit. In addition, the heat shrinkage in such integral spring units is not constant in each condition; quality control of the spring unit, necessary to obtain a uniform distance between adjacent springs, becomes cumbersome. That is to say, in such integral spring units, it is difficult to maintain a uniform distance between adjacent springs. Accordingly, the conventional spring unit is very disadvantageous in view of production and cost.